1. Field of the Invention
The present invention relates to a rolling bearing holding a main spindle of a machine tool.
2. Description of the Related Art
A main spindle of a machine tool is required to rotate with high precision and small increase in temperature for maintaining high working accuracy. High precision and a small increase in temperature during rotation are also required for a bearing supporting the main spindle. To meet these requirements, a small amount of grease and a slight amount of oil (oil mist or air oil) are employed in bearings for lubrication.
A small temperature rise in bearings for use in the main spindle of the machine tool must be maintained to prevent a degradation in working accuracy caused by the thermal deformation of the main spindle. Thus such bearings are used in the demanding condition of high-speed rotation with a small amount of lubricant oil. Under such a harsh condition, roughness and wear on the raceway caused by a slight shortage of oil film, damage such as peeling and smearing, and a shortened lifetime caused by seizing may be problems. A shortage of oil film thickness can be caused by, for example, an entry into the bearing of cutting oil or chippings of the workpiece, an increase in the working load, and heat generation in the motor. Roughness and wear in the raceway may cause noise during use and degradation in the rotation accuracy of the main spindle. As the rotation speed of the main spindle is expected to further rise in the future, it is very important to prevent the damage described above.
It is, therefore, an object of the present invention to provide a bearing that has an excellent durability and an excellent surface damage resistance when used in the main spindle of a machine tool.
In the bearing for use in a main spindle of a machine tool according to a first aspect of the present invention, at least its raceway is made of steel containing by mass, C (carbon): no less than 0.6% and not more than 1.3%; Si (silicon): no less than 0.3% and not more than 3.0%; Mn (manganese): no less than 0.2% and not more than 1.5%; P: at or less than 0.03%; S (sulfur): at or less than 0.03%; Cr (chrome): no less than 0.3% and not more than 5.0%; Ni (nickel): no less than 0.1% and not more than 3.0%; Al (aluminum): at or less than 0.050%; Ti (titanium): at or less than 0.003%; O (oxygen): at or less than 0.0015%; N (nitrogen): at or less than 0.015%; and the rest is composed of Fe (iron) and unavoidable impurities. The raceway is tempered after either quenching or carbonitriding and its surface hardness presents at least HRC (Hardness of Rockwell C-scale) 58 after tempering.
The steel of the above composition, if it is quenched and tempered, has an excellent rolling fatigue resistance even with no carbonitriding. Thus it is possible to omit the carbonitriding process and thereby reduce the manufacturing cost thereof. Although it is preferable to omit carbonitriding process in terms of a reduction in the manufacturing cost, an excellent rolling fatigue resistance can be attained by applying carbonitriding instead of quenching.
Besides, the steel of the above composition is cheaper than precipitation hardening bearing steel such as M50.
A correlation is recognized between the surface hardness of bearing components made of the steel of the above composition and rolling fatigue life: higher surface hardness is likely to provide longer rolling fatigue life. Thus the rolling fatigue life is extended in the invention by making the surface hardness HRC 58 or higher. If the surface hardness is less than HRC 58, the rolling fatigue life tends to become significantly shorter, and fluctuations in useful life increase.
The improvements disclosed in this invention provide an inexpensive and highly rolling fatigue-resistant bearing for use in the main spindle of a machine tool. The bearing for use in the main spindle of a machine tool may be an angular contact ball bearing or a cylindrical roller bearing.
The following is an explanation of the preferable range of each component contained in the steel according to the present invention. The term xe2x80x9c%xe2x80x9d as used herein means percentage by mass, unless indicated otherwise.
C: 0.6 to 1.3%
Carbon is a component essential for ensuring a strength high enough for roller bearings. In this invention, the percentage of carbon content is at least 0.6% in order to attain a predetermined hardness after heat treatment. Carbides play an important role in extending rolling fatigue life; however, it has been found that large particles of carbide are generated and then shorten the rolling fatigue life if the percentage of carbon content exceeds 1.3%. The upper limit of carbon content is, therefore, determined as 1.3%.
Si: 0.3 to 3.0%
It is preferable to add silicon because Si prevents softening at high temperatures and improves the heat resistance of bearings. The lower limit of the percentage of silicon content is determined as 0.3% because such effects do not appear if Si content is less than 0.3%. The heat-resistance of bearings is increased as Si content increases; however, if the Si content exceeds 3.0%, the effect of silicon addition reaches a maximum and workability at high temperatures and machinability decreases. Therefore, the upper limit of the silicon content is determined as 3.0%.
Mn: 0.2 to 1.5%
Manganese is an element used for deoxidation of steel and the improvement of quenching properties. Since at least 0.2% of Mn addition is required to attain such effects, the lower limit of the Mn content is determined as 0.2%. On the other hand, if more than 1.5% of Mn is contained in steel, its machinability decreases significantly. Thus the upper limit of Mn content is determined as 1.5%.
P: 0.03% or less
Phosphorus segregates in an austenite grain boundary and thereby decreases the toughness and rolling fatigue life of steel. Therefore, its content is limited to 0.03%.
S: 0.03% or less
Sulfur harms the hot working ability of steel, and decreases the toughness and rolling fatigue life of steel, forming non-metallic inclusions. Its upper limit is, therefore, determined as 0.03%. It is preferable to make the S content as low as possible since sulfur exerts such negative effects on steel. However, since sulfur has the effect of improving machinability, sulfur may be included at up to 0.05%.
Cr: 0.3 to 5.0%
Chrome is an element which plays an important part in the present invention. This element is added to steel to improve its quenching properties, increase hardness by forming carbides and extend useful life. Since steel has to contain Cr at a level of at least 0.3% to provide a predetermined amount of carbide, the lower limit of Cr content is determined at 0.3%. On the other hand, if its content exceeds 5.0%, large carbides are generated and then rolling fatigue life is shortened. The upper limit of Cr content is, therefore, limited to 5.0%.
Ni: 0.1 to 3.0%
Nickel is also an important element in this invention, preventing the change in texture during rolling fatigue at high temperatures and the decrease in hardness at high temperatures, thereby extending the rolling fatigue life of the bearing. In addition, the addition of Ni leads to higher toughness and longer life under the existence of foreign substances as well as an improvement in the corrosion-resistance. Since steel has to contain Ni at a level of at least 0.1% to attain these effects, the lower limit of Ni content is determined as 0.1%. However, if the Ni content exceeds 3.0%, a large amount of austenite remains in the steel after quenching and the predetermined hardness cannot be attained. Besides, the cost of steel rises with the addition of Ni. Thus the upper limit of Ni content is determined as 3.0%.
Al: 0.050% or less
Aluminum is used as a deoxidizer during steel manufacturing. Since Al forms oxide inclusions with a high hardness and shortens the rolling fatigue life, Al content should be reduced as much as possible. Also if Al content exceeds 0.050%, the rolling fatigue life of the bearing is significantly shortened. Therefore, the upper limit of the Al content is determined as 0.050%. When trying to reduce the Al content to less than 0.005%, the steel manufacturing cost increases. Thus the lower limit of the Al content should be 0.005%.
Ti: 0.003% or less
O: 0.0015% or less
N: 0.015% or less
Titanium, oxygen and nitrogen form oxides and nitrides in steel. Because such oxides and nitrides become non-metallic inclusions in steel working as initiation points of fatigue destruction and thereby shorten rolling fatigue life, their upper limits are determined as 0.03% for Ti, 0.0015% for O and 0.015% for N.
As a result of the effects of the addition of these alloying elements, the softening of the surface of the steel is prevented even during a localized significant temperature rise due to sliding, for example. Then the surface damage resistance of the steel is improved and its rolling fatigue life is extended.
According to a second aspect of the present invention, the steel further contains at least either 0.05 to less than 0.25% by mass of Mo or 0.05-1.0% by mass of V. Then the rolling fatigue resistance and surface damage resistance can be further enhanced.
Now the preferred levels of Mo and V contents will be described below.
Mo: 0.05 to less than 0.25%
Molybdenum improves the quenching properties of steel and prevents softening during tempering by forming a solid solution of carbides. Molybdenum is added to steel particularly because it extends the rolling fatigue life of steel at high temperatures. However, if the Mo content becomes 0.25% or higher, the steel cost rises and its machinability is significantly reduced because the hardness of steel does not decrease when a softening treatment is conducted for easy machining. Mo content is, therefore, limited to less than 0.25%. On the other hand, since a Mo addition of less than 0.05% has no effect on the carbide formation, the lower limit of Mo content is determined as 0.05%.
V: 0.05 to 1.0%
Vanadium produces fine particles of carbide, combining with carbon, and thereby contributes to the formation of fine crystals that lead to a higher strength and toughness of steel. At the same time, vanadium improves the heat resistance of steel, prevents softening after high-temperature tempering, extends rolling fatigue life and reduces the fluctuations in life. Since these effects are obtained when the V content is 0.05% or higher, the lower limit of the V content is determined as 0.05%. However, if the V content exceeds 1.0%, the machinability and hot working ability of steel decrease. Therefore, the upper limit of the V content is determined as 1.0%.
Even if a high-temperature tempering is conducted on the bearing, assuming that the bearing is heated up to high temperatures, its surface hardness becomes HRC 58 or higher as a result of the addition of these elements. Then surface damage such as peeling and smearing can be successfully prevented.
In the roller bearing according to a third aspect of the present invention, wherein the raceway has a carbonitrided surface layer containing at least 10% by volume of residual austenite. The steel of the above composition provides a surface hardness of HRC 58 or higher even if the steel is tempered at high temperatures, for example 350xc2x0 C. Such high-temperature tempering reduces the amount of residual austenite and thus provides excellent dimensional stability at high temperatures, while providing a hardness at least HRC 58. As a result, the rolling fatigue life and wear-resistance of the steel at high temperatures can be improved.
It is also possible to further improve the peeling resistance and durability by forming a carbonitrided layer which contains residual austenite at a level of at least 10% by volume on the surface of the bearing components made of the steel of the above composition. This is because such a treatment provides a high degree of surface toughness and thereby prevents the occurrence of cracks and their evolution. Namely, when the N content in the surface layer is raised by carbonitriding, an Ms point (initiation temperature of martensite transformation) of the surface layer is lowered. Then a large amount of austenite remains in the surface layer not transformed into martensite during quenching. Residual austenite has a high toughness and hardens by working, contributing to the prevention of crack generation and its evolution. In a surface layer having a low Ms point, the martensite transformation starts later than in the inside and the amount of martensite transformation becomes larger than in the inside. Then a compressive residual stress exits in the surface layer and the fatigue resistance of the surface layer is improved. As a result, peeling resistance is raised and the roller life is extended. In order to attain such effects, at least 10% by volume of residual austenite is needed in the surface carbonitrided layer. Addition of nitrogen by carbonitriding is also effective for providing a higher heat resistance and smearing resistance.
Temper toughness is referred to below. Bearings used at high temperatures are typically given the tempering treatment at a temperature higher than the temperature during operation in order to stabilize their dimensions during use. A detailed investigation into the relationship between rolling fatigue life at an operation temperature of 200xc2x0 C. and tempering hardness indicates that the rolling fatigue life of steel is likely to extend as tempering hardness is raised. Particularly, even if tempering hardness is the same, steel that has been tempered at a higher temperature has a longer life. Bearings having greater hardness after tempering at a high temperature have longer lives. Also found is that if the post-tempering hardness becomes less than HRC 58, roller life is significantly shortened and fluctuations in life increase. In order to extend the life at high temperatures and reduce fluctuations in life, it is necessary to maintain hardness at HRC 58 or higher and at the same time it is preferable to raise the tempering temperature as high as possible. For example, the tempering temperature may preferably be at least 180xc2x0 C. and not more than 350xc2x0 C. Since rolling bearings are typically used at temperatures around 100xc2x0 C. the tempering temperature should be at least 180xc2x0 C.